Mills, who returned to Sheffield after an unsuccessful move to Widnes during which he made just one appearance for the Vikings, has had surgery on a hand injury after damaging the scaphoid bone.
Coach Mark Aston told the Sheffield star: “Danny will have a cast on for eight weeks and be out for up to three months.
“It’s a disappointment because he didn’t have a pre-season with us last year. But on the plus side he can keep running and keep fit with the cast on and he could be right for the start of the league season.”
Mills scored 10 tries in 14 appearances for the Eagles last season after returning to the Don Valley Stadium on loan from Widnes.
He has signed on a permanent deal for 2010, when Sheffield will move their home games to Bramall Lane, home of Sheffield United FC.